Flow measuring systems currently often use change in electrical capacitance, electrical conductivity or electrical voltage to determine volume- and/or mass flow.
A vortex, flow measuring system comprises, for example, among other things, a bluff body, which produces, further downstream, a Kármán vortex street. The pressure fluctuations of the vortex street are registered by a sensor blade. The periodic pressure fluctuations excite the sensor blade to a periodic oscillation. The movement of the sensor blade is, for example, read out with the assistance of a piezo sensor. A disadvantage of this technology lies in the limiting of the operating temperature of the flow measuring device caused by the read-out mechanism and the necessity of an on-site electronics for reading the piezo sensor, which require complex temperature insulation and measures for explosion protection for the electronics, in order to be able to achieve operating temperatures of 400° C. or more. Vortex flow measuring systems, which function without sensor blades, and in the case of which the pressure fluctuations are registered directly by a measuring transducer placed in or on the bluff body, are also known.
Coriolis-flow measuring systems utilize phase shift of a vibrating measuring tube caused by the mass of a measured material. The accessing of the signal usually occurs via plunger coils. This reading must, among other things, be corrected for temperature.
DE 603 11 048 T2 discloses a method for manufacturing a fiber optical Fabry-Pérot interferometer. The interferometer is composed, in such case, of a pair of oppositely lying optical fiber end surfaces, which are placed on a carrier material.
DE 692 26 446 T2 discloses an apparatus, which registers the oscillations of a Coriolis-mass flow-measuring device by means of an optical fiber. For such purpose, the weakening, or the degree of weakening, of an optical signal resulting from the bending of the optical fiber is utilized.
DE 41 22 799 A1 discloses an apparatus for measuring the velocity of a fluid. This apparatus includes a transducer, which again has a rod-shaped appendage. The rod-shaped appendage is responsible for modulating an optical signal transmitted via an optical waveguide.
Due to the precision required in the construction of the measuring apparatus, or due to the danger of a contamination and/or of damage, such arrangements and manufacturing processes are time-consuming and expensive.
An option for detecting length changes is to utilize various optical methods. Known methods operate in such case on an interferometric basis. For example, by means of UV lithography, a Bragg grating can be implemented inside of an optical fiber. In the case of this method, light is radiated, broadband, into the waveguide. The Bragg grating produces a Bragg reflection at a defined wavelength. This wavelength depends equally strongly on temperature and length expansion. Thus, in the case of this method, it is not directly possible to distinguish between temperature related and length change related effects.
All of the measuring systems named have the disadvantage that, in the case of higher temperatures, a deformation of the optical resonator or of the optical reflection surfaces occurs, or the measuring transducer is even completely destroyed.
Recently, fiber optical sensors have become known, which also function at temperatures of 800° C., and thus exceed the thermal usage range of previously known sensors by several hundred degrees Celsius. For production of such sensors, a Fabry-Pérot resonator is micro-manufactured in an optical fiber by means of a laser. The facettes arising through the manufacture of the gap on the ends of the optical waveguide have mirror-like properties. The temperature independence results from the fact that the core of the optical fiber thusly manufactured expands in the case of a temperature increase, and presses the two resonance surfaces closer together. At the same time, the jacket (cladding) of the optical fiber expands and draws the resonance surfaces apart. These two effects overlap with each other and result in a small but random temperature dependence (compare Optics Letters, Nov. 1, 2007, pp. 3071-3073; Photonics Spectra, December 2007 “To boldly go where no sensor has gone before”).